Conventionally, as an effective image diagnosis apparatus for early detection of breast cancer, there is known an x-ray mammography apparatus. Further, in recent years, there is developed a photoacoustic imaging method in which a pulse laser beam is irradiated onto a living body instead of an X-ray, so as to image a three-dimensional structure inside the living body based on a photoacoustic wave induced by thermal expansion of an optical absorber inside the living body. This apparatus has received attention as a new mammography apparatus that may easily diagnose a potential tumor such as an abnormal growth of new blood vessels.
Such a mammography apparatus has a feature that an abnormal region which is difficult to detect by other apparatus may be detected as a clear image. However, for precise diagnosis of an early cancer, it is usually necessary to use both a mammography image and an ultrasonic echo image in combination. In view of this, for example, Japanese Patent Application Laid-Open No. 2006-102494 and Japanese Patent Application Laid-Open No. 2008-161283 disclose examples of an x-ray mammography apparatus incorporating an ultrasonic diagnosis apparatus. In these apparatuses, a breast as an object is fixed to a compression plate, and a mammography sensor and an ultrasonic probe are moved mechanically for scanning along the compression plate so that a mammography image and a three-dimensional ultrasonic echo image of the entire breast may be obtained simultaneously.
In obtaining three-dimensional echo image data of a wide area with mechanically scanning the ultrasonic probe, it is most advantageous in speed and cost to move a one-dimensionally arranged probe in a direction perpendicular to the arrangement direction of elements or in a direction crossing the same (hereinafter, referred to as elevation direction) continuously while obtaining the echo image data. Using the one-dimensionally arranged probe, it is possible to reconstruct a cross-sectional slice image by electronic scanning in the arrangement direction and in an ultrasonic beam emitting direction. Therefore, there may be obtained three-dimensional image data of the entire mechanical scan area by stacking the cross-sectional slice images generated at individual positions in the elevation direction.
However, this method has a problem that image resolution in the elevation direction significantly reduces as compared with image resolution in a slice plane. A first reason is that if the mechanical scan speed by the probe is decreased so as to reduce a pitch of obtaining the cross-sectional slice images, it may increase a physical load to the human object. Therefore, it is necessary to lower the pixel density in the elevation direction to some extent. In addition, a second reason is that effective angular aperture of the one-dimensionally arranged probe in the elevation direction is smaller than that in the arrangement direction, so that a reconstruction resolution in the elevation direction is deteriorated. If a two-dimensionally arranged probe is used, this problem may be alleviated to some extent. However, the number of transmission and reception elements increases, which leads to another problem that a necessary electric circuit scale increases, making it difficult to achieve the practical use of the apparatus in terms of cost.